1. Field of the Invention
The present invention relates to a radiation-curable vinyl chloride copolymer, and more particularly, to a radiation-curable vinyl chloride copolymer with good curability when irradiated with radiation.
The present invention further relates to a radiation-curable composition comprising the above copolymer, and a magnetic recording medium comprising a radiation-cured layer formed by using the above copolymer.
2. Discussion of the Background
In particulate magnetic recording media, the binder plays important roles in the dispersion of magnetic particles, coating durability, electromagnetic characteristics, running durability, and the like. Accordingly, various binders for magnetic recording media have been studied.
Vinyl chloride resins, polyurethane resins, polyester resins, acrylic resins, and other thermosetting resins and thermoplastic resins have conventionally been widely employed as binders in magnetic recording media. Moreover, the use of radiation-curable resins incorporating radiation-curable functional groups as binders for magnetic recording media has been proposed in recent years. Since processing to cure radiation-curable resins takes less time than for thermosetting resins, they are advantageous in terms of productivity. For example, Documents 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2004-352804) and English language family member US2004/241497A1 and U.S. Pat. No. 7,001,955, Document 2 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-8866), and Document 3 (Japanese Patent No. 3125947), which are expressly incorporated herein by reference in their entirety, propose the use of radiation-curable vinyl chloride copolymers as binders in magnetic recording media.
Since a magnetic recording medium is normally subjected to repeated use for extended periods, one required physical property is running durability. Accordingly, the binder for a magnetic recording medium is desirably one that forms a tough coating that is able to withstand repeated running. However, the radiation-curable vinyl chloride copolymers described in Documents 1 to 3 cure inadequately when cured with radiation and cannot form high-strength coatings.
Further, in a magnetic recording medium sequentially comprised of a nonmagnetic layer and a magnetic layer on a nonmagnetic support, the poor curability of the nonmagnetic layer may compromise electromagnetic characteristics, storage property, and running durability. This is caused by the following reason.
When sequentially applying a multilayer coating comprised of a lower layer in the form of a nonmagnetic layer coating liquid and an upper layer in the form of a magnetic layer coating liquid, a portion of the nonmagnetic layer will sometimes dissolve into the solvent contained in the magnetic layer coating liquid. When the nonmagnetic layer is a radiation-cured layer, exposure to radiation causes the radiation-curable components in the nonmagnetic layer to polymerize and crosslink, resulting in a high molecular weight and thereby inhibiting or diminishing dissolution into the solvent contained in the magnetic layer coating liquid. Accordingly, when sequentially applying a multilayer coating comprised of a lower layer in the form of a nonmagnetic layer coating liquid and an upper layer in the form of a magnetic layer coating liquid, radiation is desirably irradiated before applying the upper layer in the form of the magnetic layer coating liquid, after which the magnetic layer is desirably formed over the cured nonmagnetic layer. However, when the nonmagnetic layer cures poorly, even when the magnetic layer is formed over a nonmagnetic layer that has been cured with radiation, it is difficult to adequately inhibit interlayer mixing due to dissolution of the nonmagnetic layer into the magnetic layer coating liquid. As a result, interfacial fluctuation increases between the nonmagnetic layer and the magnetic layer and thus the smoothness of the surface of the magnetic layer decreases, compromising electromagnetic characteristics.
Further, when there is inadequate curing of the nonmagnetic layer, large amounts of nonmagnetic layer components migrate to the magnetic layer side. This results in a large amount of various components seeping out from the surface of the magnetic layer. When such a phenomenon occurs, the storage property of the medium deteriorates because the medium sticks during storage, precipitates form on the medium surface, and the like. Further, when curing of the nonmagnetic layer is inadequate, magnetic layer components tend to permeate the nonmagnetic layer. As a result, the magnetic layer becomes nonuniform and the coating strength of the magnetic layer decreases, thereby compromising running durability.
As set forth above, the radiation-curable resin that is employed in the nonmagnetic layer is required to be highly curable. However, as stated above, the radiation-curable vinyl chloride copolymers described in Documents 1 to 3 do not adequately cure when exposed to radiation and do not afford adequate characteristics for use as binders in the nonmagnetic layer.